1. Technical Field
The present invention relates to color mapping techniques in general, and, in particular, to a method and apparatus for converting a color representation of an image to a grayscale representation.
2. Description of Related Art
Quite often, color graphical objects, such as pie charts and bar charts, need to be converted to monochrome prints. The term “monochrome” refers to a single color of predominantly the same hue, and the most common example is grayscale. Color to grayscale conversion is an important conversion for both monochrome and color printers.
Typically, color can be described by lightness, chroma and hue. The most common method of performing color to grayscale conversion is to use a device independent color space, such as CIElab color space, as the intermediate color space to discard the chroma and hue information, and only the lightness information are converted to grayscale. Such conversion is essentially a lightness mapping. However, the well-known Kelmholtz-Kohlrausch effect indicates that at the same lightness level, the perceived lightness increases with increasing saturation. Thus, one drawback of the lightness mapping approach is that the perceived color differences in the original colors are not faithfully represented in the output. In other words, colors with large hue differences but small lightness differences will be indistinguishable in the output even though they were quite distinguishable in the input. As a result, a pie chart having red, green, yellow, etc. could show up a gray circle after the color to grayscale conversion.
Consequently, it would be desirable to provide an improved method and apparatus for converting a color representation of an image to a grayscale representation while retaining the discriminability of the original colors.